ICMCTF2011 Session A1-2: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

While the exhaust in natural gas-fired land based turbines contains 10-15% water vapor, the exhaust with coal-derived fuels or innovative turbine concepts for more efficient carbon capture may be 30-85%. To investigate the effect of increased water vapor levels on thermal barrier coating (TBC) lifetime, furnace cycling tests were performed at 1150°C in air with 10, 50 and 90 vol.% water vapor. The specimens were all from the same batch of a commercial second generation superalloy and had Pt diffusion or Pt-modified aluminide bond coatings and commercially vapor-deposited yttria-stabilized zirconia top coats. The lifetime results will be compared to a prior study where similar coatings were thermally cycled in dry oxygen.

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Research sponsored by the U. S. Department of Energy, Office of Fossil Energy, Coal and Power R&D.

1. Cheng, et al. J Phys D: Appl. Phys. 42 (2009)1.

2. Cheng, et al., Surf. Coat. Technol. 204(2010)2123.

Delayed failure of TBCs is a widely observed phenomenon, even though many of the occurrences are anecdotal and unreported. The phenomenon is characterized by survival of a TBC after considerable thermal cycling and full cooling to room temperature. Yet, after some extended time under ambient conditions, the TBC may be found to be nearly fully delaminated. This has been termed “the weekend effect” or “desktop spallation.” It is generally assumed to result from exposure to ambient humidity, which may be considerably increased compared to the furnace environment. To further demonstrate the humidity factor, oxidized TBCs were subject to water immersion or a water drop exposure after being retained and fully cooled from a thermal cycling treatment. It is often found to then fail quite dramatically in less than a second, after some 10s of seconds of incubation time. To this end, digital video recordings have become a most useful observation technique. We report on early “no bond coat” plasma sprayed coatings, where 1100-1150^oC cyclic life and resistance to water immersion are seen to increase as the sulfur content of the superalloy substrate was reduced. EB-PVD coatings using a Pt-aluminide bond coat are also seen to fail by the water drop test after 1150^oC cyclic furnace tests. More recently, water drop failure is observed for a commercial TBC coated turbine blade, after oxidation at 1150-1200^oC. In support of this phenomenon, colleagues from DECHMA (Rudolphi, Renusch, Schütze) and CNRS Toulouse/SNECMA (Déneux, Cadoret, Hervier, Monceau) have both produced very compelling and informative studies. They have used video, acoustic emission, and extended dry box storage of oxidized TBCs.

The phenomenon is rooted in moisture-induced delayed spallation of the alumina scale formed on the bond coat. In that regard, many studies show susceptibility of alumina scales to moisture, as long as high strain energy and a partially exposed interface exist. The latter result from severe cyclic oxidation conditions, producing a highly stressed and partially damaged scale. In one model, it has been proposed that moisture reacts with aluminum in the bond coat to release hydrogen atoms that ‘embrittle’ the interface. A negative synergistic effect with interfacial sulfur is also invoked.

A new approach to manufacture a complete thermal barrier system in a single step is being studied during the European FP7 project called PARTICOAT. Spherical Al particles are deposited by screen-printing and air brush on IN738, Rene80 and CM247 nickel based alloys. During the sintering process in air the Al particles are partially oxidized and converted into hollow alumina spheres forming a ceramic “foam” (top coat), and simultaneously an Al rich diffusion layer (bond coat) is formed in the subsurface zone of the substrate.

The coatings were isothermally exposed at 800 and 1000°C in air for up to 1000 hours. The oxide formation and the microstructure of the coating were studied by thermo gravimetrical analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX).

The coatings were adherent for all the tested substrates and temperatures. The CM247 alloy shows the lowest mass gain whereas IN738 and Rene80 show higher mass gains at the tested temperatures. The use of reactive element oxides in the coating has also been tested, showing an improved bond-coat microstructure in the case of HfO₂. These results demonstrate the flexibility and viability of this low cost coating concept.